The present invention relates to an air-fuel ratio (A/F) closed-loop control method and apparatus for an internal combustion engine.
It is a well-known practice to provide an internal combustion engine with an A/F closed-control system. Such system calculates the A/F correction factor in response to a detection signal from a concentration sensor. The sensor detects the concentration of a particular component contained in the exhaust gas. An example of such a sensor is an oxygen (O.sub.2) sensor for detecting the concentration of oxygen in the exhaust gas. The A/F closed-loop control system corrects the feeding rate of fuel supplied to the engine according to the calculated correction factor so that the engine A/F is the desired value.
Such A/F closed-loop control system is provided in general with a lean monitor. The lean monitor monitors whether or not the O.sub.2 sensor's output is continuously maintained at a voltage which indicates that the engine A/F is on the lean side with respect to the stoichiometric condition for a time longer than a predetermined period. Namely, the lean monitor compares the O.sub.2 sensor's output with a reference signal and produces a malfunction signal if the O.sub.2 sensor's output is continuously below the reference signal for a time longer than a predetermined period. As is known, the O.sub.2 sensor's output is continuously held at a small voltage for a long period if the O.sub.2 sensor is in an inactive state or if breaks occur in the connector or the wiring of the O.sub.2 sensor or in the O.sub.2 sensor itself. Therefore, by means of the lean monitor, the inactive state of the O.sub.2 sensor and the occurrence of breaks in the O.sub.2 sensor can be detected.
In the conventional lean monitor, however, as the reference signal voltage for comparison is fixed at a predetermined voltage, breaks in the O.sub.2 sensor or in the connector or the wiring of the O.sub.2 sensor sometimes cannot be detected. This is because the O.sub.2 sensor's output changes depending upon the engine rotational speed so that the higher the rotational speed, the smaller the O.sub.2 sensor's output and vice versa. If the reference signal voltage for comparison is determined at a voltage which is appropriate for detecting breaks and whether or not the O.sub.2 sensor is in an inactive state when the rotational speed of the engine is low, the O.sub.2 sensor's output is always below the reference signal voltage when the rotational speed is high. Accordingly, if breaks occur at high rotational speed they cannot be detected.